1. Field of the Invention
The present invention relates to a method of producing semiconductor element which is able to enhance the dielectric strength of the insulation film such as a tunnel oxide film of nonvolatile semiconductor memory, and the same memory produced by this method.
2. Description of Related Art
To begin with, giving brief description about nonvolatile semiconductor memory which is the background of the present invention: as shown in FIG. 10 and FIG. 11, this kind of memory is able to memorize one bit of data by accumulating a group of electrons in a floating gate 9. According to FIG. 10, when data is written in the memory, the gate is impressed with a voltage. Thus, the channel is opened. And hot electrons come out of the channel. And these electrons are accumulated in the floating gate 9. On the other hand, as shown in FIG. 11, when data in the memory is erased, the source is impressed with a voltage in stead of the gate. Thus, the electrons accumulated in the floating gate 9 is let out by tunnel current from the floating gate 9 to the control gate 11. In this operation, as shown in FIG. 11, the tunnel current flows through a sharp portion of the floating gate 9 (see e.g. U.S. Pat. No. 5,067,108 Jenq).
Tunnel oxide film of this kind of semiconductor memory is a very thin dielectric with thickness of less than 100 angstroms (c.f. Jenq). As an actual method to form this kind of thin film, there is a chemical vapor deposition (CVD) which uses mixture gas of SiH4 and N2O. The conditions of film forming are the temperature 750° C., the pressure 0.45 Torr, and the atmosphere SiH4:N2O=24:1200 cc. And after film forming, the film formed is annealed. The conditions of annealing are the temperature 1000° C., the atmosphere N2, and the process time 30 seconds.
In the semiconductor memory mentioned above, it is necessary to keep the dielectric strength of the very thin tunnel oxide film in high level. Otherwise, the electrons which should be kept in the floating gate 9 become likely to leak to the control gate 11 through the sharp portion of the thin film shown in FIG. 11. Even if the source is not impressed with the high voltage.
Accordingly the inventor of the present invention tested the dielectric strength of the sample of tunnel oxide film which was formed with the conventional method. The result is shown in FIG. 9. The method of the test is to prepare a sample of prescribed thickness and sectional area. This sample is impressed with a voltage at the thickness. The voltage is gradually increased. Then the leak current is detected. And a sample of thermal oxide film was tested with the same method. Both of the leak currents were compared. Then the dielectric strength of the CVD tunnel oxide film was evaluated under the standard of the thermal oxide film.
Both results of the test of these samples are shown in FIG. 9. Samples which thickness is about 100 angstroms were prepared for both of conventional CVD oxide film and thermal oxide film. And the voltage impressed at thickness of each sample is increased from 0V. Then the next phenomena appeared. About the conventional CVD oxide film, when the voltage was 10V, a leak current of 1.E-12A flowed. So the dielectric began to be broken at this moment. And the strength of the electric field that is the dielectric strength at this moment was 10V÷100 Å=10 MV/cm. On the other hand, about the thermal oxide film, when the voltage was 12V, a leak current of 1.E-12A flowed. So the dielectric began to be broken at this moment. And the strength of the electric field that is the dielectric strength at this moment was 12V÷100 Å=12 MV/cm. As a next step, about the conventional CVD oxide film, when the voltage was 12V, a leak current of 1.E-10A flowed. A current of CVD oxide film flowed at the voltage of 12V was 100 times larger than that of thermal oxide film at same voltage. After this, each voltage impressed to both samples was increased. Then each current flowed as represented in the characteristic curve in FIG. 9. As shown in FIG. 9, the characteristic curves of the voltage versus current have similar figures about both kinds of samples. And it has been revealed that the average current of CVD oxide film which flows at each voltage is some 10 times larger than that of thermal oxide film. In addition, the vertical coordinate in FIG. 9 has a logarithmic scale. By this method, testing samples of oxide film, the tunnel oxide film which dielectric strength is near to that of thermal oxide film is obtained. And the subject of this invention is to obtain tunnel oxide film which dielectric strength is as high as possible.